The production of microemboli and altered left ventricular (LV) function was studied in 24 adult sheep during 4 hours of cardiopulmonary bypass (CPB), using membrane (12 sheep) and bubble oxygenators (12 sheep). Arterial pressure, HR, CVP, and dP/dt were comparable in the two groups. Active compliance curves the LV intracavitary balloon volume vs. filling pressure were determined every 30 minutes. Ultrasonic on-line counts of particulate and bubble microemboli were made at 3 sites: venous line, distal to oxygenator, and arterial line. At the onset of CPB, bubble oxygenators produced 62,000 microemboli/liter of flow (74% bubbles, 26% particles) and membrane oxygenators produced 1360 microemboli/liter of flow (88% bubbles, 12% particles). Over 4 hours of CPB,, embolization 71% in the bubble oxygenator group secondary to particles; whereas, embolization 61% in the membrane group. LV dV/dP was 1.6 cc/mmHg at the onset of CPB, and 42% in the bubble group and 23% in the membrane group over 4 hours (P .05). We conclude that bubble oxygenators produce more microemboli than membrane oxygenators do, and that these microemboli may contribute to a decrease in LV compliance during prolonged CPB.